It is a potty that your tamagotchi is sitting on. It is using that instead of relieving itself in the bottom of the screen.
If you catch your tama on the floor with those wavy lines by it, click the pooper scooper icon. If it is a toddler, your tama will be on the duck potty. But if it is a teen or an adult, it will be on the toilet.
If you catch your tama in time to make it do it's business in the potty or toilet, a happy heart will fill up. Your tamagotchi will most likely be hungry after relieving itself, so make sure to check the stats and give it what it needs!
GotchiGirl96